1. Field of the Invention
This invention relates to power supplies. More specifically, this invention relates to circuits which drive power switching transistors used in switching regulator and DC/DC converter type power supplies.
While the present invention is described herein with reference to a particular embodiment in a particular application, it is to be understood that the invention is not limited thereto. Those having ordinary skill in the art to which this invention pertains will recognize modifications and other applications within the scope of the invention.
2. Description of the Prior Art
Voltage regulating power supplies generate single or multiple output DC voltages for a given load within specified limits. Simple resistive series and/or shunt regulating type power supplies were frequently used highly inefficient regulating techniques. In fact, until high current, low forward drop transistors became available, these series or shunt resistive regulating schemes were typically used up to power levels of one thousand watts.
To improve the efficiency of power supplies, switching mode power supplies, such as switching regulators and DC to DC converters, were developed. Switching regulator power supplies, for example, achieves higher efficiencies by incorporating highly efficient switching devices to periodically couple input power to an intermediate reactive storage element which in turn provides relatively constant power to a system load. Instead of absorbing the voltage difference between the input and the desired output with a power dissipating element, the low impedance transistor switch is made to open and close periodically between input and output. The switch output varies between approximately 0 volts and the input voltage. The reactive energy storage element provides the average or DC value of this input.
A DC to DC converter functions as a DC transformer to efficiently change from one DC voltage level to another. This conversion usually is accomplished by using switching mode power transistors to convert the input DC voltage to an AC square wave and then converting this square wave to a higher or lower voltage amplitude via the turns ratio transformation of a power transformer. The transformer output square wave is then recitified and filtered to generate a new DC voltage level usually having a higher or lower voltage or a different ground reference than the input DC voltage power level.
The following patents disclosed illustrate prior art regulating schemes.
______________________________________ Pat. No. Patentee Refer To ______________________________________ 3,040,183 Farnsworth FIG. 1 3,076,135 Farnsworth et al FIG. 1 3,174,094 Farnsworth et al FIG. 1 3,569,818 Farnsworth et al FIG. 1 4,069,449 Farnsworth FIG. 1 ______________________________________
See also Switching and Linear Power Supply, Power Converter Design by Abraham I. Pressman, published by the Hayden Book Company, 1977, pages 321-325.
In both switching regulators and DC/DC converters, the power transistor switch needs a base driving circuit to provide the specialized turn on and turn off waveforms as determined by the transistor specifications. Bipolar switching transistor specifications typically set forth three important characteristics for such a base drive circuit.
(1) Steady state (DC) ON base current should be adequate to keep the transistor in heavy saturation for the highest steady state collector current load. This base drive current should not, however, increase to excessive values during worst case conditions.
(2) At the instant of turn on the base drive should be much higher than the steady state value; i.e., a base current peaking pulse is required. The base current peaking pulse provides the necessary collector current to charge the collector circuit capacitance and other transient collector loads. For switching transistors the relationship between base current and collector current is given by the formula: EQU .sub.c =(.beta..sub.S)(I.sub.B)
where
I.sub.c is collector current PA2 .beta..sub.S is saturated beta PA2 I.sub.B is forward base current.
(3) At the instant of turn off the switching transistor base to emitter voltage should reverse polarity by several volts in order to achieve the transistor manufacturer's guaranteed minimum switching times.
While high steady state base drive is typically easy to obtain, the simplest DC prior art base drive schemes do not achieve the base current peaking nor the required reverse base to emitter voltage drive for optimum switching transistor operation. Establishing such base current peaking and reverse base drive using prior art techniques is somewhat expensive in components and space.
One prior art technique for providing reverse base drive uses a coupling transformer in the base drive circuit with a magnetizing inductance so low that its core absorbs adequate energy from the driving source during ON time and uses it during OFF time to provide reverse base drive for fast turnoff.
This reverse base drive through transformer coupling has several disadvantages. For example, the coupling transformers work best when provided with square wave or sine wave inputs having a uniform duty cycle. Resetting the core with nonsymmetrical waveforms requires more complicated techniques. Yet such nonsymmetrical waveforms are likely in switching regulators where the duty cycle of the base drive signal may vary depending upon the unregulated input voltage and the load voltage.
A second disadvantage of reverse base drive through transformer coupling is that a steady state (DC) ON or OFF base drive signal cannot be applied to the switching transistor through the coupling transformer without the use of complicated 2 phase schemes. This disadvantage is particularly undesirable in a voltage regulator insofar as there are situations in which it may be necessary to have a switching transistor in its ON state for a substantial period of time or in its OFF state for a substantial period of time.
A third disadvantage of transformers as reverse base drive mechanisms is that the transformers are custom components which are expensive to specify, design, and manufacture.